1. Field of Invention
This invention relates generally to aircraft brake assemblies and more particularly to devices and methods for protecting aircraft brake assemblies from the deleterious effects of runway de-icer compositions and cleansing liquids.
2. Description of Related Art
Runway de-icers are used at airports during the winter months to ensure that the runway surfaces are maintained free of ice in the interest of aircraft safety. De-icers based on glycol liquids and urea have traditionally been used. These materials have been found to present a hazard to the environment (see Jane's Airport Review July/August 1996, page 42) and more environmentally benign formulations based on acetates and formates have recently been introduced.
It has been realised that exposure of carbon-carbon composites, e.g. brake discs, to contaminants such as runway de-icers and aircraft cleaning fluids can cause an increase in the oxidation rate of the composite at temperatures above about 450° C. There has been a marked increase in the oxidation rate since the introduction of environmentally friendly runway de-icers. A small amount of oxidation causes a dramatic reduction in the material strength and can even result in the failure of drive tenons, especially from the stator discs. Oxidation weight loss of only 5% can result in a loss of strength of approximately 25%.
Carbon-carbon composite materials will oxidise at temperatures above 450° C. in the absence of contact with de-icer or cleaning solutions but the oxidation rate is controlled by anti-oxidative protection applied to discs and is not found to be a problem during the service life of the brake. The relationship between the oxidation problem and de-icers is evidenced by the fact that the problem is worse in winter months than in summer months and in the case of aircraft regularly operating out of airports in cold climates.
Carbon-carbon composite brake discs are exposed to the runway de-icer spray as the aircraft takes off, lands and during taxiing. The spray runs into the brake assembly after splashing onto the exposed parts of the brake, such as the cylinder block, or spray containing the de-icer can be drawn into the brake by the airflow caused by cooling fans that cool the brakes after landing. The de-icer solution can seep into pores within the composite where it dries onto the material when the discs are at an elevated temperature, thereby contaminating them. This contamination can then act as a catalyst to accelerate oxidation at temperatures in excess of about 450° C., resulting in a reduction in service life of the brake discs.
EP-A1-0555822 describes the use of a cylindrical louvered heat shield mounted so as to rotate with an associated wheel. The heat shield is intended to prevent ingress of foreign objects, protecting an inner heat shield and the heat stack. Any impinging liquid is thrown off of the louvered heat shield as a consequence of its rotation.
It is one object of the invention to provide means to ameliorate or avoid the reduction in service life of carbon-carbon composite brake discs which results from contact with substances such as de-icers and cleaners that are used in airports, such as on runways.